Detecting robot stasis

ABSTRACT

A coverage robot includes a drive configured to maneuver the robot as directed by a controller, a stasis indication wheel rotatable about a first axis perpendicular to a direction of forward travel, and a suspension supporting the wheel. The stasis indication wheel defines a first reflective portion and a second reflective portion. The second reflective portion is substantially less reflective than the first reflective portion. The suspension permits movement of the wheel in a direction other than rotation about the first axis. A signal emitter is disposed remotely from the wheel and positioned to direct a signal that sequentially is intercepted by the first and second reflective portions of the wheel. A signal receiver is positioned to receive the reflected signal by the rotating wheel. Communication between the emitter and the receiver is affected by rolling transitions between the first and second reflective portions during permitted movement of the wheel.

CROSS-REFERENCE TO RELATED APPLICATIONS

This U.S. patent application claims priority under 35 U.S.C. §119(e) to U.S. provisional patent application 60/803,608, filed on May 31, 2006, the entire contents of the which are hereby incorporated by reference.

TECHNICAL FIELD

This disclosure relates to detecting a stasis condition of a coverage robot.

BACKGROUND

In the field of home, office, and/or consumer-oriented robotics, mobile robots performing household functions involving traversing a floor or other surface to vacuum cleaning, floor wash, patrol, or cut a lawn, have become commercially available. Machines for clearing debris from carpet or hard floors, for example, may traverse a carpet or hard floor surface, for example. In the course of moving over the floor or other such surface, these robots may odometrically monitor the distance they travel, or determine whether they are in fact moving, by counting the number of, or monitoring, revolutions made by a rotatable wheel that remains in frictional contact with the surface while the robot moves over the surface.

SUMMARY

In one aspect, a coverage robot includes a drive configured to maneuver the robot as directed by a controller in communication with the drive, a stasis indication wheel rotatable about a first axis perpendicular to a direction of forward travel, and a suspension supporting the wheel. The stasis indication wheel defines a first reflective portion and a second reflective portion. The second reflective portion is substantially less reflective than the first reflective portion. The suspension permits movement of the wheel in a direction other than rotation about the first axis. A signal emitter is disposed remotely from the wheel and positioned to direct a signal that sequentially is intercepted by the first and second reflective portions of the wheel. A signal receiver is positioned to receive the signal as reflected by the wheel as the wheel rotates with respect to the emitter and as the suspension permits movement of the wheel in a direction other than rotation about the first axis. Communication between the emitter and the receiver is affected by rolling transitions between the first reflective portion and the second reflective portion of the wheel in multiple positions about the first axis and along permitted movement in a direction other than rotation about the first axis.

In another aspect, a coverage robot includes a drive configured to maneuver the robot as directed by a controller in communication with the drive, a stasis indication wheel rotatable about a first axis perpendicular to a direction of forward travel, and a suspension supporting the wheel. The stasis indication wheel includes a hub and multiple spokes extending outwardly from the hub. The stasis indication wheel defines a first reflective portion and a second reflective portion. The second reflective portion is substantially less reflective than the first reflective portion. A signal emitter is disposed remotely from the wheel and positioned to direct a signal that sequentially is intercepted by the first and second reflective portions of the wheel. A signal receiver is positioned to receive the signal as reflected by the wheel as the wheel rotates with respect to the emitter and as the suspension permits movement of the wheel in a direction other than rotation about the first axis. Communication between the emitter and the receiver is affected by rolling transitions between the first reflective portion and the second reflective portion of the wheel in multiple positions about the first axis and along permitted movement in a direction other than rotation about the first axis.

Implementations of the disclosure may include one or more of the following features. In some examples, the permitted direction of wheel movement other than rotation about the first axis includes vertical displacement of the wheel. The coverage robot may include a drop wheel sensor arranged to detect vertical displacement of the wheel. The permitted direction of wheel movement other than rotation about the first axis may also include tilting away from a vertical second axis perpendicular to the first axis, rotation about a vertical second axis perpendicular to the first axis, and translation parallel to an underside of the robot. In some implementations, the coverage robot includes a wheel housing secured to the suspension and shrouding the wheel. The wheel housing defines an aperture in a top portion of the wheel housing, exposing the wheel to the signal emitter and the signal receiver. The wheel housing is configured to minimize an entry of ambient light into the aperture of the housing.

In some examples, the reflective portion of the wheel reflects light of a first infrared wavelength and the non-reflective portion reflects light of a second infrared wavelength different from the first wavelength. The emitter and receiver may be arranged above the wheel, the emitter emitting the signal onto a top portion of the wheel, and the receiver responsive to reflected emissions.

In some implementations, the coverage robot includes a cliff signal emitter and a cliff signal receiver. The cliff signal emitter is aligned to emit a cliff detection signal onto a floor surface adjacent the wheel. The cliff signal receiver is configured to receive the cliff detection signal reflected from the floor surface. The cliff signal emitter and cliff signal receiver are arranged with respect to the floor surface such that communication between the cliff signal emitter and the cliff signal receiver is affected by vertical movement of the floor surface with respect to the robot.

In some implementations, the controller monitors a drive current of the drive to determine a presence of a stasis condition. For example, if the drive current increases significantly, it may be indicative of a robot stuck condition. If the drive current drops significantly, it may be indicative of a free wheeling condition, as when a drive wheel is off a cliff edge.

In some examples, the stasis indication wheel comprises a hub and multiple spokes extending outwardly from the hub. The stasis indication wheel may include a magnet disposed on one of the spokes. The magnet interacts with a reed switch in communication with the controller to provide wheel rotation or traction feedback. In some examples, the spokes of the stasis indication wheel are compliant. The stasis indication wheel may be disposed adjacent a drive wheel.

The details of one or more implementations of the disclosure are set fourth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1A is an above-perspective view of a coverage robot.

FIG. 1B is a below-perspective view of a coverage robot.

FIG. 2 is a perspective view of a wheel having cams, illustrating a cam engaging the leaf switch.

FIG. 3 is a perspective view of a wheel having cams, illustrating the leaf switch in the non-engaged state.

FIG. 4 is a schematic view of a caster wheel assembly with a bi-colored wheel.

FIG. 5 is a perspective view of an axle arm assembly for mounting a wheel.

FIG. 6 is a bottom schematic view of a robot incorporating a caster wheel assembly.

FIG. 7 is a top view of the axle arm assembly for mounting a wheel, illustrating the swiveling ability of the axle arm assembly about a Z-axis.

FIG. 8 is an oblique view of a bi-colored wheel and optical sensor, with the optical sensor facing a darkened portion of the bi-colored wheel.

FIG. 9 is an oblique view of a bi-colored wheel and optical sensor, with the optical sensor facing a lightened portion of the bi-colored wheel.

FIG. 10 is a schematic perspective view of an axle arm assembly for mounting a wheel retained by an axle arm base.

FIG. 11 is a top view of an axle arm base.

FIG. 12A is a perspective view of a caster wheel assembly having an optical stasis detection system.

FIG. 12B is an exploded view of a caster wheel assembly having an optical stasis detection system.

FIG. 13A is a section view of the caster wheel of FIG. 12, view along line 13-13.

FIG. 13B is a side view of the caster wheel and an optical stasis sensor assembly.

FIG. 14A is an exploded perspective view of an optical stasis sensor assembly.

FIG. 14B is a front view of a back housing of an optical stasis sensor assembly.

FIG. 15 is a schematic view of a drive motor current stasis detection system.

FIG. 16 is a perspective view of a drive wheel assembly.

FIG. 17 is a schematic view of a magnet/electrically biased wheel stasis detection system.

FIG. 18A-18B are bottom schematic views of robots incorporating a stasis indicator wheel assembly.

FIG. 19A-19B are perspective views of stasis indicator wheels.

FIG. 20 is a perspective view of a stasis indicator wheel assembly and a drive wheel assembly.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

A cleaning robot may, for example, include a front wheel which is not motor driven, but instead spins freely as robot drive wheels (or other powered drive system, such as treads, legs, or directed air jets, inter alia) propel the robot. Such a free-spinning wheel may be used as part of a robot stasis detection system (and/or distance, motion or speed detection system), in which the spinning of the wheel is used to gauge whether or not the robot is in fact moving. Such a distance, motion, speed, or stasis detector may be used to detect if the robot is jammed against an obstacle or if the drive wheels are disengaged from the floor, for example, as when the robot is tilted or becomes stranded on an object. In such circumstances, the drive wheels may spin when the mobile robot applies power to them, but the robot is not moving.

FIG. 1A is a top perspective view showing an example of a coverage robot 100. The coverage robot 100 may be used to clean a work surface, such as a floor or wall by vacuuming debris brushed from the work surface. The robot 100 includes a chassis 102 which carries an outer shell 104, a drive system 110, a main cleaning head 120, an edge cleaning head 130, and a controller 140. Drive system 110 is mounted on the chassis 102, and is a differential drive (left and right wheels near to or on the center diameter of the robot 100 and independently speed controllable) configured to maneuver the robot 100. The main cleaning head 120 is carried by the chassis 102 and includes a main brush 122 and a flapper roller 124, both rotatably coupled the housing of the main cleaning head 120. The edge cleaning head 130 is mounted to extend past the side edge of the chassis 102 for removing dirt and debris below and immediately adjacent to the robot 100, and more particularly to sweep dirt and debris into the cleaning path of the main cleaning head 120 as the robot 100 cleans in a forward direction. In some implementations, the main or edge cleaning heads 120, 130 may also be used to apply surface treatments. A front bumper 108 is secured to the chassis 102 and may contain contact sensors. The controller 140 (also depicted in FIG. 6) is carried by chassis 102 and is controlled by behavior based robotics to provide commands to the components of robot 100 based on sensor readings or directives to clean or treat floors in an autonomous fashion. A battery 109 may provide a source of power for robot 100 and its subsystems.

FIG. 1B is a bottom perspective view showing an example of the coverage robot 100. The drive system 110 includes a left drive wheel assembly 112, a right drive wheel assembly 114 and a caster wheel assembly 116. The drive wheel assemblies 112, 114 and the caster wheel assembly 116 are connected to the chassis 102 and provide support to the robot 100. The controller 140 may provide commands to the drive system 110 to drive wheels 112 and 114 forward or backwards to maneuver robot 100. For instance, a command may be issued by the controller 140 to engage both wheel assemblies 112, 114 in a forward direction, resulting in forward motion of robot 100. In another instance, a command may be issued for a left turn that causes the left wheel assembly 112 to be engaged in the forward direction while the right wheel assembly 114 is driven in the rear direction, resulting in the robot 100 making a clockwise turn when viewed from above.

Referring to FIG. 2, in some implementations, a wheel stasis detector system 200 includes a cammed wheel 201 having cams 202 disposed about a periphery of the cammed wheel 201. The cams 202 repeatedly engage and disengage a leaf contact 213 of a leaf switch 212 of a sensor circuit 210. An electrically conducting portion 204 of the cammed wheel 201 may provided a voltage via a rotating electrical connector (e.g. a brush contact 222 which has electrically conductive contact fibers 223 that brush against a metallic surface 204 of the cammed wheel 201). A constant-voltage DC power supply 226 forms a brush circuit 220 with the conductive portion 204 and the wheel 201 in combination with a resistance 228, for example.

As the cammed wheel 201 spins during robot movement across a floor, the cams 202 repeatedly trigger the leaf switch 212. The leaf switch 212 in turn may be connected to ground through a second resistance 218 which shares a common node with the output of the leaf switch 212, for example, and to which a monitor or control circuit 214 may also be connected. As the cammed wheel 201 rotates and the leaf switch 212 is repeatedly triggered, a train of electrical pulse signals is generated, which the monitor 216 can interpret as evidence of ongoing motion of the robot 100.

The arrangement shown in FIGS. 2-3 may involve complex wire routing and may encounter difficulty in placing switches in sufficient proximity to the cams 202 of the cammed wheel 201, for example. The leaf switch 212 employs a delicate leaf contact 213, which is rapidly and repeatedly physically deformed numerous times as the cammed wheel 201 spins, making it prone to wear and breakage (and may be particularly vulnerable to damage if debris contacts the leaf contact 213). The cammed wheel 201 is typically rigidly fixed in a set alignment, resisting rotation about a Z axis, thereby not contributing to directional stability of the robot 100 through swiveling. Deviation from its fixed alignment would generally removes the cams 201 from contact with the leaf contact 213 (and therefore trigger a “false positive” detection of stasis).

FIG. 4 illustrates a caster wheel assembly 300 including a stasis indication wheel 302, which may be a load-bearing wheel for supporting the weight of the robot 100; or, alternatively, the stasis wheel 302 may be a trailing or non-load-bearing wheel. The assembly shown in FIG. 4 is “upside-down,” i.e., the wheel 302 would ordinarily be suspended from the caster frame 360 (held within or integral with the robot 100 body or chassis 102. The stasis indicating wheel 302 is rotatably coupled to a wheel housing 310 and may spin freely in response to frictional contact with a surface or floor during robot movement. The stasis wheel 302 includes at least one lightly colored (or reflective) section 304 and at least one darkly colored (or non-reflective) section 306. The wheel housing 310 is supported by a suspension 315. The suspension 315 may be any device from which a mechanical part is suspended (in this context, the mechanical parts including a wheel on the underside or undercarriage of a robot 100), that optionally permits one additional degree of freedom (rotational or translational, “additional” meaning in addition to driving or driven rotation of the wheel for movement along a surface) for the robot 100 and/or for the suspended part; and/or a system of springs and/or other devices that transmits, attenuates, or cushions vibration or shock between a vehicle and its wheels. In the present case, the suspension 315 can permit one additional degree of freedom in the form of steering rotation of the wheel 302 about the Z-axis (with the wheel 302 rotating in a driven or driving rotation about an arbitrary X-axis) without having any cushioning function, but may also have a cushioning function (damped or resilient translation along z axis) or wheel drop functions (translation biased down along z axis) or additional degrees of freedom as described herein. In some implementations, the suspension includes a shaft/spindle 312 carrying a spring 314. The shaft 312 is rotatably and slidably connected to a wheel mount 340 secured to the robot chassis 102. The shaft 312 can rotate about and slide along its longitudinal axis. The spring 314 biases the wheel 302 and wheel housing 310 away from the chassis 102. A wheel drop sensor 342 carried by the wheel mount 340 (as shown) or arranged near the wheel to detect vertical wheel displacement. Examples of the wheel drop sensor 342 include a switch activated by the shaft 314 and/or an infrared emitter and detector pair arranged to detect vertical wheel displacement (which may be the optical sensor 320, if the field of view and emission of the sensor 320 are arranged to terminate in a manner that prevents signal returns, e.g., if the wheel 302 has a light section 304 and dark section 306 darker than the light section 304 and providing contrast therewith, yet the absence of a dropped wheel 302 from the field of view and/or emission of the optical sensor 320 appears with an even lower IR reflection than the dark section 306—i.e., three decreasing levels of reflectivity—a first level from the light section 304, a lower second level from the dark section 306, and a yet lower third level, including a null level, from the absence of a dropped wheel 302). The robot 100 includes an optical sensor 320 having a field of vision (i.e. a field of view if the optical sensor 320 depends on ambient light; or both a field of view and field of emission if the optical sensor 320 includes an illuminating emitter) trained on the stasis wheel 302. The optical sensor 320 detects a contrast transition or a transition from the light section 304 to the dark section 306 (or vice versa) within the field of vision of the optical sensor 320. When the optical sensor 320 detects such a transition, a signal is transmitted to the controller 140, which can perform an appropriate analysis on the received data. Alternatively, the controller 140 itself can interpret the time-dependent signal from the optical sensor 320 and perceive either contrast transition or discrete level changes.

In some examples, the controller 140 calculates the number of transitions detected by the optical sensor 320 during a period of time and determines based on pre-set or dynamically calculated parameters whether or not the robot 100 is moving (and/or a robot speed). When no transitions are detected, the controller 140 concludes that the robot 100 has ceased moving relative to the floor. If the diameter of the stasis wheel 302 is fixed and known to the controller 140, the controller 140 may calculate a distance traversed by the robot 100 by multiplying the number of transitions detected by the wheel diameter and a transformation constant (similarly, the robot speed may be calculated by dividing a particular period of time by the distance calculated for that period of time).

Referring to FIG. 5, in some implementations, the stasis wheel 302 is connected to an axle or axle arm 330, about which the wheel 302 can spin (e.g. one suspension discussed herein). The axle arm 330 may include a portion for mounting to the chassis 102 of the robot 100. In one example, the axle arm 330 includes an upper cap 332 and lower cap 334 for holding the axle arm 330 to the wheel mount 340.

FIG. 6 provides a schematic view of the robot 100 including the caster wheel assembly 300 mounted near the front of the robot 100. The stasis wheel 302 serves as a swiveling caster wheel positioned under the front portion of the robot 100. By mounting the stasis wheel 302 to the robot 100 as a caster, the directional and/or steering stability of the mobile robot 100 may be enhanced. The stasis wheel 302 (also referred to hereinafter as a “bi-colored wheel”) may be a non-powered follower wheel, which can spin freely as the robot 100 is propelled by the drive system 110, which may be separate from the caster wheel assembly 300.

In some implementations, the caster wheel assembly 300 includes a motor to drive the stasis wheel 302, for example, when the weight of the robot 100 generates sufficient frictional force with surface being traversed that the wheel 302 cannot spin if the robot 100 is not actually moving, even when the wheel 302 is being actively powered by a motor or other drive system.

The transitions between the light section 304 and the dark section 306 of the wheel 302 can be detected by the optical sensor 320 despite swiveling of the wheel 302. Therefore, the wheel 302 may serve both as an enhanced load-bearing wheel of the robot 100 and simultaneously as part of the motion, speed, or stasis detection system 300 of the robot 100.

In some implementations, the optical sensor 320 includes a laser-based or mouse-like optical camera (which may have a resolution of about 100 pixels, for example) in which the focal length of the camera is selected to span from substantially ground level to the camera, with a depth of field within the a limited range of travel or the full range the robot suspension (e.g., 1-2 cm). The field of vision of the optical sensor 320 may include both the bi-colored wheel 302 and a portion of the ground over which the robot 100 traverses. Output from the optical sensor 320 may be used by the robot controller 140 to determine the rate at which the wheel 302 is rotating (e.g. by counting the number of black/white transitions within a particular period of time) and also track actual movement of the robot 100 with respect to the cleaning surface by analyzing and reckoning a rate and/or direction of travel based on the image of the ground recorded by the optical sensor 320. The robot 100 can therefore compare wheel movement to actual surface-relative movement and detect whether the wheel 302 has lost effective contact with the surface (e.g. when the robot 100 determines a discrepancy between the actual rate of travel and a wheel speed).

FIG. 7 illustrates the stasis wheel 302 swiveling and a field of vision of the optical sensor 320. The optical sensor 320 has a field of vision which extends over a wide area, allowing the optical sensor 320 to detect transitions as the stasis wheel 302 spins even while swiveling about a Z-axis (i.e., the vertical axis orthogonal to the plane of the floor or surface to be swept) across a wide angular range. The permissible range of swivel for the stasis wheel 302 can be set as wide as the field of vision of the optical sensor 320 can detect. Alternatively, for example, two or more optical sensors 320 may be positioned at different angles surrounding the range of swivel of the stasis wheel 302, such that the colored portions 304, 306 of the stasis wheel 302 are within the useful field of vision of at least one of the optical sensors 320, permitting a swivel range as wide as 360 degrees.

Referring to FIGS. 8-9, when the stasis wheel 302 spins so as to transition from the dark section 306 to the light section 304 within the field of vision of the optical sensor 320, an output signal sent from the optical sensor 850 may transition from a first voltage level to a second voltage level. An edge transition between the first and second voltage states may be detected by the controller 140; or, as an alternative, the optical sensor 320 may perform the edge detection and only output a pulse signal to the controller 140 when the transition is detected. Although FIGS. 8-9 show a solid-circle stasis indicating wheel 302, the term “wheel” includes spoked, perforated, armed, pinwheel, or other wheels as discussed herein.

Referring to FIGS. 10-11, in some implementations, the wheel 302 is mounted to an axle arm base 400 via the axle arm 330. The axle arm base 400 is secured to the chassis 102 and includes a rounded hub 410 (and, in some examples, a corresponding bottom hub (not shown)) over which the top cap 334 of the axle arm 330 is placed. A concavity 420 (shown to have a generally parabolic profile) defined by the hub 410 allows the axle arm 330 to swing/tilt in relation to the axle arm base 400. The tiltable axle arm 330 can affect a castered orientation (i.e., tilted or deviating from the vertical Z-axis by an angle

c) relative to the axle arm base 400. The stasis wheel assembly 300 provides stability to the robot 100 by virtue of having a caster wheel 302 that swivels and tilts, allowing the wheel 302 to trail the front of the moving robot 100. A second hub (not shown), inverted relative to the hub 410 on the top side of the base 400, may also be positioned on the bottom side of the base 400 and interact with the lower cap 332 to provide a similar effect.

Referring to FIGS. 12A-13B, in some implementations, the caster wheel assembly 300 includes a bi-colored wheel 302 mounted to a caster hub 350, which in turn is mounted to a caster frame 360. The stasis wheel 302 is rotatably coupled to a wheel housing 310 and may spin freely in response to frictional contact with a floor surface during robot movement. The wheel housing 310 may have a rounded shape (e.g. a half-teardrop shape), permitting it to slide over obstacles. The wheel housing 310 defines a wheel cavity 311 which houses the bi-colored wheel 302. The stasis wheel 302 includes at least one lightly colored section 304 and at least one darkly colored section 306. The wheel housing 310 is supported by a shaft/spindle 312 carrying a spring 314. The shaft 312 is rotatably and slidably connected to a wheel mount 340 secured to the robot chassis 102. The shaft 312 can rotate about and slide along its longitudinal axis in the caster hub 350, allowing the wheel 302 to swivel freely and be displaced in the vertical direction. When the stasis wheel 302 is positioned in the wheel housing 310, a locking piece (or any other suitable retaining structure) may secure the stasis wheel 302 for rotation in the wheel housing 310.

As shown, the wheel housing 310 defines a leading rounded ramp or slope that intersects the surface of the stasis wheel 302 at less than an angle of 45 degrees from a tangent to the stasis wheel 302 at a point of intersection. Specifically, in the direction of forward travel, a leading slope or ramp of the housing, extending from a point lower than the center axis of the wheel 302 to approximately the full wheel height readily seen in FIGS. 13A-13B as a gentle slope 20-40 degrees from horizontal (including curved slopes that transition from to a steeper slope from bottom to top), encounters obstacles before the wheel 302 and may overcome them, including obstacles of higher than ½ the wheel height. Alternatively, any suitable profile and corresponding wheel cavity 311 may be used. The wheel housing 310 may be substantially opaque to prevent external light from illuminating a sampling portion 303 of the stasis wheel 302 exposed outside the wheel housing 310 and facing the optical sensor 320.

In some examples, the caster frame 360 is configured to allow the received caster hub 350 to tilt away from a vertical position within the caster frame 360. A hub spring may bias the caster hub 350 to a neutral vertical position. The spring 314 on the shaft 312 compresses when the stasis wheel 302 bears the weight of the robot 100, thereby pressing the wheel housing 310 toward the caster frame 360. If the wheel housing 310 abuts the caster frame 360 under the weight of the robot 100, the wheel housing 310 (and therefore also the wheel 302) can still swivel about the longitudinal axis 313 defined by the shaft/spindle 312.

If the robot 100 encounters a cliff or overhang (or steep grade) in the surface it is traversing, the stasis wheel 302 may drop and eventually lose contact with the surface. When such a situation occurs, the spring 314, normally partially compressed by the weight of the robot 100 above, urges the wheel housing 310 and wheel 302 away from the caster frame 360. A wheel- or hub-distance sensor (e.g. a mechanical switch or a break-beam sensor) may then signal to the controller 140 that the robot 100 has encountered a drop-off and trigger an appropriate behavioral reaction, such as causing the robot 100 to attempt to back up, for example. In some examples, the caster frame 360 houses a cliff sensor 370 including an infrared emitter 372 and corresponding detector 374. The cliff sensor pair 370 is arranged to sense a surface presence adjacent the stasis wheel 302. The cliff sensor pair 370 detects an encountered cliff, steep down grade, or some other edge when infrared light emitted by the emitter 372 is no longer reflected off a ground surface back to the detector 374. The cliff sensor pair 370 may also be configured to detect vertical wheel movement.

As shown in FIGS. 13A-13B, light provided by the optical sensor 320 strikes the sampling portion 303 of the wheel 302, thereby reducing undesirable interference or false signals from external light sources and increasing the reliability of the optical sensor 320 in detecting transitions between the light area 304 and the dark area 306 as the wheel 302 spins. The optical sensor 320 sensor is attached to the caster frame 360 at an angle β of about 10-40 degrees between longitudinal axis 321 defined by a sensor housing 326 of the sensor 320 and a longitudinal axis 313 defined by the shaft/spindle 312, allowing the emitter to 322 to be positioned forward or behind of the yaw/z axis of the caster wheel 302 and/or forward or behind the wheel 302 itself, yet direct a field of view and emission substantially at top dead center of the wheel 302 to emit a signal (e.g. infrared light) onto the sampling portion 303 of the stasis wheel 302 and receive a reflection therefrom. If the wheel 302 is configured to as a compliant suspension with a full range of loaded travel below 1 cm and above ⅕ cm, and/or additionally or alternatively configured to unloaded, drop a predetermined distance (e.g. about 1 to 2 cm for detecting cliff edges), emissions from the emitter 322 are still incident on the sampling portion 303 and reflected emissions are still received by the receiver 324 over the range of ordinary suspension travel and/or in some cases when the wheel 302 has dropped.

Referring to FIGS. 13B and 14A-14B, in some implementations, the optical sensor 320 includes a housing 326 having front and back portions, 326A and 326B respectively, configured to house a light source 322 that emits light and an optical receiver 324 that detects light emitted by the light source 322 and reflected by the light area 304 of the wheel 302. The housings 326A and 326 are preferably opaque to light. The receiver 324 is positioned to precede the emitter 322 in the direction of travel, and as shown in FIG. 13B is directed toward a field of view encompassing top dead center of the wheel 302, in a direction less than 15 degrees from perpendicular to the wheel 302 tangent at that position. The front and back portions, 326A and 326B respectively (in this context not “front” and “back” in the direction of robot travel), of the sensor housing 326 are configured to hold the emitter 322 and receiver 324 angled with respect to each other at an angle (φ of about 10+/−5 degrees, preferably about 10 degrees. The emission path of the emitter 322 and the receiving path of the receiver 324 meet in a field of view/overlap zone about 1-4 cm, preferably about 1 cm from the emitter 322 and/or receiver 324. The view/overlap zone encompasses the region of the wheel 302 top dead center surface when the robot 100 is on the floor, and this zone is sized (i.e., depth of effective detection) to be about at least ½ to 2 times, the loaded ordinary suspension travel of the caster wheel 302 (e.g., loaded travel being about ⅕ to 1 cm, preferably less than 2 cm) and vertically centered in an idle loaded position. Within the ordinary range of vertical wheel travel and/or when a wheel drop occurs, the wheel 302 remains in the view/overlap zone for stasis detection. If stasis detection is to be performed for a dropped or nearly dropped wheel but there is little or no ordinary suspension travel (e.g., for a vertically fixed caster), the zone depth may be ½ to 2 times the maximum wheel drop amount, vertically centered in the middle of the drop range. The principle of operation with respect to reflectivity detection, noise rejection, field of view and emission may be that disclosed in U.S. Pat. No. 7,155,308, herein incorporated by reference in its entirety.

The dark area 306 of the wheel 302 absorbs the light when facing the optical sensor 320, thereby preventing light from reflecting back to the optical sensor 320. The caster frame 360 is configured to permits the light from the light source 322 to reflect off of the light area 304 of the wheel 302 and be detected by the optical receiver 324 over a range of swivel angles of the wheel 302. The optical receiver 324 may be aimed or positioned so as to face a side-face of the wheel 302 (that is, facing either of the two broad circular sides of the wheel 302), or to face the wheel 302 edge-on (that is, facing the narrow area of the outer edge of the wheel 302 without either broad side of the wheel 302 being within the “field of vision” of the optical receiver 324, for example). As another alternative, the optical receiver 324 may be obliquely aimed toward the wheel 302.

In some examples, the optical sensor 320 may function to detect transitions between the light area 304 and the dark area 306 of the wheel 302 as the wheel 302 spins, even over a range of 40 degrees of hub swivel away from a straight wheel alignment (i.e., when the robot is moving forward without turning), by keeping the wheel 302 within the effective line of sight the optical sensor 320. The wheel housing 310 can prevent outside light from reflecting off the wheel 302 and reaching the optical receiver 324, improving the reliability of the optical receiver 324.

Referring to FIGS. 15-16, a transition algorithm may be employed by the controller 140 to transition from a first stasis detection system to a second stasis detection system, when the efficacy of the first stasis detection system degrades below a threshold level. In some examples, the robot 100 is equipped with a first stasis detection system 300, as described above, having a bi-colored swiveling caster wheel 302 with light sections 304 and dark sections 306, in which an optical sensor 320 detects transitions from the light section 304 to the dark section 306 (and vice versa) as the bi-colored wheel 302 spins. In addition, the robot 100 includes a second stasis detection system 500 for detecting motion, speed, or stasis of the robot 100. The second stasis detection system 500 includes a drive motor current sensor 510 which monitors the current (herein the “drive current”) drawn by a drive motor 502 that turns one or more drive wheels 508 for propelling the robot 100. The drive motor current sensor 510 and the drive motor 502 are both carried by a drive wheel assembly housing 505. When the drive current is higher than a threshold value, the second stasis detection system 500 determines that the robot 100 is in a stasis condition (e.g. a condition typically occurring when the drive wheels are spinning, but the robot 100 is not moving because it is blocked by an obstacle, or mired in some other way). By monitoring the contrast between the detection of the light and dark sections, 304 and 306 respectively, of the bi-colored wheel 302, the optical sensor 320 can output a signal to the controller 140 indicating that the bi-colored wheel 302 has become too dirty or obscured to be useful in motion, speed, or stasis detection, for example.

The robot controller 140 may use an algorithm to transition from the first stasis detection system 300 (the bi-colored wheel 302 and optical sensor 320) as the primary sensor used for motion, speed, or stasis detection to the second stasis detection system 500 (the drive current sensor 510). The transition may be unitary (i.e., switching entirely and immediately), or it may be gradual in degree and/or time (e.g. by applying a confidence coefficient for the first stasis detection systems 300 and/or the second stasis detection system 500). The robot controller 140 may evaluate the inputs from both stasis detection systems 300, 500 simultaneously, integrating the evaluated inputs according to an integration equation, in accordance with the nature of the first and second stasis detection systems.

Referring to FIG. 17, in some implementations, the wheel 302 includes a magnetic marking 804 on one half, the other half 806 being not magnetized. A reed switch, or Hall effect detector 820 detects the proximity of the magnetized half 804 of the wheel 302 as the wheel 302 rotates, in a manner generally similar to the optical detector 320 discussed above. As an alternative, the magnetized portion 804 of the wheel 302 may cover less than one half of the wheel 302. The magnetic marking 804 may include a small dot of magnetized resin covering a 1 cm area positioned toward the edge of the wheel 302. In another example, the wheel 302 includes an electrode portion 804 positioned on the wheel 302, in which an electrical charge is applied to the electrode 804 when the robot 100 is in operation. In this configuration, a detector 820 (e.g. an inductive, capacitive electrical detector) is positioned such that the charged electrode 804 on the wheel 302 passes by the detector 820, generating a signal received by the controller 140.

Referring to FIGS. 18A-20, in some implementations, the robot 100 includes a stasis indicator assembly 400 including a stasis indication wheel 402. FIG. 18A-18B are bottom schematic views of robots 100 incorporating a stasis indicator wheel assembly 400. FIG. 19A-19B are perspective views of stasis indicator wheels 402. The stasis indication wheel 402 may be a load-bearing wheel for supporting the weight of the robot 100; or, alternatively, the wheel 402 may be a non-load-bearing wheel. The stasis indication wheel 402 includes a rotatably hub 403 and spokes/arms 405 extending outwardly from the hub 403. In some examples, the spokes 405 spiral outward from the hub 403. The spokes 405 may be compliant in order to generate enough traction to turn the wheel 402 but not take away from traction of a driven wheel 113 of the drive wheel assembly 112, 114. The wheel 402 is rotatably coupled to a wheel housing 410 and may spin freely in response to frictional contact with a surface or floor during robot movement. The wheel 402 includes at least one lightly colored (or reflective) section 404 and at least one darkly colored (or non-reflective) section 406. In some implementations, the wheel 402 defines a pinwheel shape and is formed as a half (or quarters, or eighths, or per spoke) reflective solid, from compliant rubber or the like, and is snapped past a rim and onto a stationary axle of the wheel housing 410 with a loose slip fit, to rotate freely on the axle. The spokes 405 are spirals or helixes ending in an inwardly pointing distal elbow tip at about 90%, which limits the compliance/travel range of the spokes 405 (more rigid materials would use longer distal elbow portions). A spring effect/constant of each spokes 405 is cumulative when one spoke elbow abuts the next spoke 405. In order to bend further, both spokes 405 must bend against increasing resistance. This effect is limited to two or three successive spokes 405. Most of the spoke bending of takes place in the middle.

The robot 100 shown in FIG. 18B includes only two driven wheels 113, lacks a trailing or leading caster, and uses a cleaning head 120 as a third point of contact for stable mobility. The robot 100 includes an optical odometer/stasis “pinwheel” 400 substantially aligned with and adjacent one of the two differential driven wheels 113, all on a center axis 101 defined by the robot 100. This configuration may be used with robots lacking a leading and/or trailing caster (because there is no caster to monitor for stasis) or with a leading and/or trailing caster (in order to avoid adding instrumentation to the caster).

In some implementations, the wheel 402 and associated wheel housing 410 are disposed adjacent one of the left and right drive wheel assemblies, 112 and 114 respectively. In one example, the wheel housing 410 may supported by a suspension 315. The suspension may include a shaft/spindle 312 carrying a spring 314. The shaft 312 is rotatably and slidably connected to a wheel mount 340 secured to the robot chassis 102. The shaft 312 can rotate about and slide along its longitudinal axis. The spring 314 biases the wheel 402 and wheel housing 410 away from the chassis 102. In another example, as shown in FIG. 20, the wheel housing 410 is integral with a housing 111 of the drive wheel assembly 112, 114 and the stasis indication wheel 402 moves in unison with the driven wheel 113 of the drive wheel assembly 112, 114, which may include a suspension. Having the wheel 402 coupled to each driven wheel 113 of both drive wheel assemblies 112, 114 allows software on the controller 140 to compensate for wheel slippage and provide traction feedback for generating advanced escape behaviors of the robot 100. In some implementations, the stasis indicator assembly 400 includes a magnet placed within one of the spokes 405 to generate rotational feedback the controller 140 via a reed switch.

A number of implementations have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the disclosure. Although reference has been made to cleaning and/or vacuuming robot by way of examples, it is nonetheless understood that any of the features set forth may also apply to any suitable type of robot or mobile machine which utilizes a free-turning wheel as part of a motion, speed, or stasis-detection system. Accordingly, other implementations are within the scope of the following claims. 

What is claimed is:
 1. A coverage robot comprising: a drive configured to maneuver the robot as directed by a controller in communication with the drive; a swivel caster assembly coupled to the drive and comprising: a stasis indication wheel freely rotatable about a horizontal axis parallel to a traveling surface, the stasis indication wheel defining a first reflective portion and a second reflective portion, the second reflective portion being substantially less reflective than the first reflective portion; a wheel housing mounted on a freely rotatable shaft positioned at an angle relative to the horizontal axis, the wheel housing supporting the wheel such that rotation of the shaft causes the wheel to swivel on the traveling surface; and a suspension supporting the wheel housing and permitting movement of the wheel in a direction other than rotation about the horizontal axis and swiveling on the traveling surface; a signal emitter disposed remotely from the wheel and positioned to direct a signal that sequentially is intercepted by the first and second reflective portions of the wheel; and a signal receiver co-located adjacent the signal emitter to receive the signal as reflected by the first and second reflective portions of the wheel as the wheel freely rotates about the horizontal axis as the wheel swivels on the traveling surface, and as the wheel moves in the direction permitted by the suspension, wherein the strength of the reflected signal received by the receiver changes based at least in part on rolling transitions between the first reflective portion and the second reflective portion of the wheel in multiple swivel positions and along the movement permitted by the suspension.
 2. The coverage robot of claim 1 wherein the permitted direction of wheel movement by the suspension comprises vertical displacement of the wheel.
 3. The coverage robot of claim 2 further comprising a wheel drop sensor arranged to detect vertical displacement of the wheel.
 4. The coverage robot of claim 1 wherein the permitted direction of wheel movement by the suspension comprises tilting away from a vertical axis.
 5. The coverage robot of claim 1 wherein the permitted direction of wheel movement by the suspension comprises translation parallel to an underside of the robot.
 6. The coverage robot of claim 1 wherein the wheel housing shrouds the wheel and defines an aperture exposing the wheel to the signal emitter and the signal receiver.
 7. The coverage robot of claim 6 wherein the wheel housing is configured to minimize an entry of ambient light into the aperture of.
 8. The coverage robot of claim 1 wherein the first reflective portion of the wheel reflects light of a first infrared wavelength and the second reflective portion reflects light of a second infrared wavelength different from the first wavelength.
 9. The coverage robot of claim 1 wherein the emitter and receiver are arranged above the wheel, the emitter emitting the signal onto a top portion of the wheel, and the receiver responsive to reflected emissions.
 10. The coverage robot of claim 1 further comprising a cliff signal emitter and a cliff signal receiver, the cliff signal emitter aligned to emit a cliff detection signal onto the traveling surface, the cliff signal receiver configured to receive the cliff detection signal reflected from the traveling surface; wherein the cliff signal emitter and cliff signal receiver are arranged with respect to the traveling surface such that communication between the cliff signal emitter and the cliff signal receiver is affected by vertical movement of the traveling surface with respect to the robot.
 11. The coverage robot of claim 1 wherein the controller monitors a drive current of the drive to determine a presence of a stasis condition.
 12. The coverage robot of claim 1 wherein the stasis indication wheel is disposed adjacent a drive wheel.
 13. A coverage robot comprising: a drive configured to maneuver the robot as directed by a controller in communication with the drive; a stasis indication wheel freely rotatable about a horizontal axis parallel to a traveling surface and freely rotatable about a vertical axis perpendicular to a direction of forward travel and disposed adjacent the drive, the wheel comprising a hub and multiple spokes extending outwardly from the hub, the wheel defining a first reflective portion and a second reflective portion, the second reflective portion being substantially less reflective than the first reflective portion; a signal emitter disposed remotely from the wheel and positioned to direct a signal that sequentially is intercepted by the first and second reflective portions of the wheel; and a signal receiver positioned to receive the signal as reflected by the first and second reflective portions of the wheel as the wheel freely rotates about the horizontal and vertical axes, and as a suspension permits movement of the wheel in a direction other than rotation about the vertical axis, wherein the strength of the reflected signal received by the receiver changes based at least in part on rolling transitions between the first reflective portion and the second reflective portion of the wheel in multiple positions about the vertical axis and along permitted movement in a direction other than rotation about the vertical axis.
 14. The coverage robot of claim 13 wherein the spokes of the stasis indication wheel spiral outwardly from the hub.
 15. The coverage robot of claim 13 wherein the spokes of the stasis indication wheel are compliant.
 16. The coverage robot of claim 13 wherein the stasis indication wheel includes a magnet disposed on one of the spokes, the magnet interacts with a reed switch in communication with the controller.
 17. The coverage robot of claim 13 wherein the first reflective portion of the wheel reflects light of a first infrared wavelength and the second reflective portion reflects light of a second infrared wavelength different from the first wavelength.
 18. The coverage robot of claim 13 wherein the emitter and receiver are arranged above the wheel, the emitter emitting the signal onto a top portion of the wheel, and the receiver responsive to reflected emissions.
 19. The coverage robot of claim 13 wherein the controller monitors a drive current of the drive to determine a presence of a stasis condition.
 20. The coverage robot of claim 1 wherein the shaft is freely rotatable within a predetermined angular range as wide as the field of detection of the signal receiver, such that the signal receiver receives the reflected signal irrespective of a swivel position of the wheel on the traveling surface.
 21. The coverage robot of claim 1 wherein the signal receiver is a first signal receiver, and further comprising a second signal receiver, the first and second signal receivers having a sufficient aggregate field of detection to receive the reflected signal throughout 360 degrees of rotation by the shaft. 